These weapons were originally designed
to arm the new carriers being built in the 1930s but they eventually became
the standard weapon for destroyers built late in World War II and rearmed
several capital ships as well.

The Mark I and Mark III guns were used
in twin mountings and were interchangeable with each other, the only difference
being in details of the firing mechanism. Several ships actually
carried both gun types, depending upon what was available when they were
being built or modernized. The Mark IV was ballistically identical
to the Mark I and Mark III guns but slightly modified in order to fit on
the standard 4.7" (12 cm) CP XXII single
mountings used on destroyers. The Mark II was an Army AAA weapon,
generally similar to the naval guns but not mounted afloat.

Following the failure of the 5.1"/50
(13 cm) QF Mark I, the 4.5" (11.4 cm) caliber was selected in the middle
1930s for a new DP weapon for carriers, as it was believed that this was
the largest caliber that could be used for a fixed round. The complete
round for this weapon weighed 85 lbs. (38.6 kg) versus 108 lbs. (49 kg)
for the complete round of the 5.1" (13 cm) gun. However, even though
this was a lighter round than that for the 5.1" (13 cm) gun, the decision
on the basis of weight was contradictory for two reasons. First,
the slightly lighter 4.7" (12 cm) ammunition for destroyers had always
been made in separate form in order to reduce the task of the ammunition
handlers. Second, a fixed round had been designed in the 1920s for
the 4.7"/40 (12 cm) Mark XII anti-aircraft
gun used on the Nelson class battleships and this weighed 74 lbs. (33.6
kg) complete. During service evaluations, it was found that this
weapon could not maintain a high rate of fire - a necessity for an AA weapon
- as the heavy round rapidly wore out the gun crews. So, if by the
early 1930s, it was being found that a 74 lbs. (33.6 kg) fixed round was
too heavy to allow sustained firing, then it is odd that a few years later
an 85 lbs. (38.6 kg) fixed round was deemed to be acceptable. It
would appear that the active Navy and the Admiralty had a considerable
lack of communications regarding practical experience with fixed ammunition.

Twin mountings used on carriers and
capital ships built before and during World War II fired fixed ammunition.
As noted above, this fixed round proved to be too heavy in service use
and there was a tendency for the projectiles to separate from the cartridge
cases during normal handling. These problems resulted in a rapidly
decreasing rate of fire during prolonged firing periods. The mountings
for the post-war built carriers Ark Royal and Eagle were modified versions
using separate ammunition.

On 8 May 1941 while Renown was engaging
Italian torpedo bombers, a fail-safe lockout system malfunctioned and her
P3 mounting fired into the back of P2 mounting, killing six and wounding
twenty-six crewmembers. Angle iron frameworks were later erected
around the mountings to prevent a repeat episode. The fail-safe gear
was overhauled during a refit at Rosyth later that same year.

In 1944, guns of 4.7" (12 cm) caliber,
which had been used on nearly every British destroyer built since 1918,
gave way to a new destroyer weapon, the 4.5" (11.4 cm) QF Mark IV gun.
It had originally been planned that destroyers would also use fixed ammunition,
but reports from the cruisers Scylla and Charybdis told of loading problems
during rough weather due to the heavy weight of the ammunition. As
these were 6,000 ton (6,100 mt) cruisers, it was an obvious conclusion
that the much lighter destroyers would have even worse handling problems
- a conclusion that the Royal Navy had been aware of for over a decade.
To alleviate the problem, a new 4.5" (11.4 cm) gun designed to use separate
ammunition was developed for arming destroyers. Compared to the standard
4.7"
(12 cm) Mark IX, this new destroyer weapon had a higher rate of fire
and used a heavier projectile with better ballistic properties, but it
is questionable if these advantages outweighed the disadvantage of adding
yet another mid-caliber weapon and its specialized munitions to the logistical
system. It did have a post-war advantage in that this new caliber
became the standard size for Royal Navy destroyers and continues in use
to the present day.

Nomenclature note: In the 1950s
the British weapon designation system changed from being per the gun itself
to being per the mounting. At the same time, arabic numerals replaced
roman numerals. Some confusion was created under this new system
because older weapons were redesignated, even though the weapons and mountings
themselves did not change. Under this new system, the "4.5-in (11.4
cm) Mark I, Mark III and Mark IV guns" in the "Mark II, Mark III, Mark
IV and Mark V mountings" were re-designated as "Mark 2" and "Mark 3" for
the twin mounts used on carriers and as "Mark 4" (twin) and "Mark 5" (single)
for the mounts used on destroyers. Asterisks used in the old designation
system to denote changes were carried forwards to the new system, but new
changes were given "Mod" numbers. This meant that the Royal Navy
now had designations such as "4.5-in Mark 5* Mod 2" which meant that this
particular mounting had gone through three modifications since it was first
designed. As could be expected, these changes have led to much confusion
as to what weapons were actually used on any particular ship. For
this reason, at the top of this datapage, I show both the original per-the-gun
designation and, in parenthesis, the per-the-mounting redesignations.

Gun construction was of an autofretted
loose barrel, jacket, removable breech ring and sealing collar. The
breech mechanism used a horizontal sliding breech block with hand-operated
closing and semi-automatic opening. Mark I had both percussion and
electric firing while the Mark III was designed for all-electric firing.
The Mark I was later modified to permit electric only firing. The
Mark IV used electric firing and the breech mechanism was considerably
lighter with more generous radii on some components. Five experimental,
46 Mark I, 524 Mark III and at least 199 Mark IV guns were manufactured.
Guns needed to be dismounted in order to change the barrels. It should
be noted that these weapons had a relatively short barrel life.

All British 4.5" naval guns have an
actual bore diameter of 4.45" (11.3 cm).

4.5" Mark II BD mountings on HMS RenownNote the safety rails, which were erected
after an accident killed six crewmen and wounded many morePhotograph copyrighted by E.J. Smith (Leading
Steward 1940-42)

1) Outfits were SAP and HE for battleships
and aircraft carriers but only HE for depot ships. By the end of
the war, most propellant charges were flashless, if available, and SAP
and HE were carried by all except aircraft carriers, which had HE only.
Except in depot ships, 25% of the HE was to have VT proximity fuzes, rising
to 50% as availability increased.

2) Destroyers during the war were also
equipped with 10 rounds per gun of a RDX/TNT filled HE for use against
surfaced submarines.

3) Star shell allowances during the war
were 150 to 400 rounds per ship but depot ships had none.

4) Projectiles were 5/10crh.

5) The outfit for Renown per gun was 250
HE rounds, 150 SAP rounds, 75 practice HA rounds and 25 practice LA rounds.
It is believed that this was the same outfit as carried by Queen Elizabeth
and Valiant. The outfit for the early Battle class was 460 SAP, 720
HE, 100 Starshell, 160 LA practice and 160 HA practice.

.

Range

.

Elevation

With 55 lbs. (25 kg) HE Shell

Range @ 43 degrees

19,900 yards (18,200 m)

Range @ 45 degrees

20,750 yards (18,970 m)

AA Ceiling @ 50 degrees(Max elevation of single mounting)

29,910 feet (9,120 m)

AA Ceiling @ 80 degrees(Max elevation of twin mounting)

41,000 feet (12,500 m)

.

Armor Penetration with
55 lbs. (25 kg) SAP Shell

.

Range

NC Armor

Deck Armor

10,500 yards (9,600 m)

2.5" (76 mm)

---

Note: Data from "British
Battleships of World War Two." Assumes a perpendicular impact.

1) All of the Mark II-type mountings were
cramped and difficult to maintain. These were BD (Between Deck) mountings
that had only a low shield above the weather deck with the bulk of the
mounting below it. The original Mark II had a 46 hp motor and oil
hydraulic pump in the fixed structure. The guns were not individually
sleeved and run out was by compressed air. The elevation arcs were
driven by pinions on a common shaft and powered by a hydraulic motor.
Training was by hydraulic motor driving a worm gear. The arrangements
for the RP10 Mark II** differed in having a vertical roller race to limit
jump and using rack and pinion gears in the power elevation and training
rather than worm gears. The ammunition supply in Ark Royal (1937)
was by two-stage electrically driven chain hoists, with the first stage
running from the handling rooms to the upper deck and the second stage
running from the upper deck to the gun positions. The Illustrious
class had similar dual hoists, but on the upper deck there were horizontal
conveyors which carried the shells over to the loading positions for the
upper chain hoists. For the Implacable class, the magazines were
located directly under the gun positions and so no horizontal conveyors
were needed. The modernized battleships and battlecruiser used a
single stage hoist that brought rounds up into the superstructure.
For all ships, these hoists or conveyors did not come into the gunhouse.
Instead, a human chain was used to carry the rounds from the hoists to
a three-round scuttle on each side of the rear of the mounting. From
here, the rounds were passed by hand to a fuze setting machine, one for
each gun, and then onto the loading tray. The loading trays for most
Mark II mountings were rotated by hand about a tubular shaft from the cradle
and contained the hydraulic and hand rammers. For the Mark II mountings,
the round was grasped by two crewman, one at each end, and together they
would lift the round into the loading tray and then push the tray over
to behind the breech. The shell would then load automatically.
The loading trays on the RP10 Mark II** were power-operated. Ark
Royal (1950) was equipped with Inclined Duplex Endless (IDE) chain hoists,
one for each mounting. These hoists ran directly from the magazines
to the mountings and could deliver thirty rounds per minute. Ark
Royal (1950) was unique in that she used USN Mark 37 GFCS instead of HACS
for gunnery control.

4) The Mark IV was a true turret mounting
that externally resembled the RP10 Mark II** but differed in having a revolving
stalk. The prototype was installed on HMS Savage, where it proved
fairly successful. This mounting-type was then used on the Battle-class
destroyers. The mountings on the Battle class could make one and
a half revolutions in each direction from the mid (fore and aft) position
for a total of 1080 degrees of train. The barrels shared a common
slide and each had its own ammunition supply. Each gun had a hydraulic
pusher shell and cartridge hoists on the revolving structure and extended
one deck down to where they were fed by hand from the fixed electric endless-chain
hoists. Both hoists automatically stopped operating when the "top
of the hoist" position was full, but immediately restarted when the ammunition
was removed. Shells were passed via the fuze-setting machines to
the loading trays, while the cartridges came up vertically to the rear
of the gun. The loading trays could be charged at any angle.
Rammers were hydraulically operated with a single self-resetting lever
control. A joystick was fitted for local control. This mounting
proved satisfactory in service, but one defect found in the trial mounting
on HMS Savage, oil leakage through the decks, was never really solved and
continued to plague at least one Battle right up until she finally paid
off in 1962.

5) The RP10 Mark IV and RP10 Mark IV* were
RPC equipped and had separate shell and cartridge hoists. The Mark
IV* was equipped to use the USA Mark 37 FCS with a Metadyne Mark VII fuze
setting machine.

6) Mark V and Mark V* mounts were converted
from 4.7" (12 cm) CP XXII mountings.
Besides the differences needed for mounting the 4.5" (11.4 cm) gun, these
converted mountings had a shorter recoil distance and were slightly lighter.
Although billed as "dual-purpose," the reality was that these mountings
had the same +55 degree maximum elevation of the previous 4.7" (12 cm)
guns which had always been seen as too low for defense against aircraft.
These mountings were also hard to work at high elevations, which slowed
their rate of fire against aircraft. The destroyer "X" mounting had
modified stops which allowed a total training arc of about 580 degrees.
This is why many photographs of these ships show this mounting stowed pointing
dead ahead.

7) The RP50 Mark V mount used electric
training and elevation Metadyne RPC equipment. A joystick was supplied
for local control.

8) The single-gun RP50 Mark V* mounting
on the last eight Battle class destroyers was equipped to use the USA Mark
37 FCS and had an improved spring-powered loading tray. Previously,
although ramming was spring powered, the tray itself had to be manually
pushed into line with the breech and then manually withdrawn after ramming.
The Mark V* had extra springs which allowed the complete loading cycle
to be automatically initiated by a single control lever. The recoil
from firing the gun recocked all spring mechanisms. This mounting
was used on the Battle class mainly for firing starshell, but SAP and HE
rounds were also supplied. This mounting also gave these ships an
after-firing gun, the lack of which was perceived as being a handicap on
the earlier ships of this class. The Metadyne Mark VII fuze-setters
used for these mountings were "not particularly successful."

9) The two Dido class cruisers armed with
these guns were originally to have had the 5.25"
(13.4 cm) Mark I but production difficulties with these weapons greatly
delayed deliveries and the 4.5" (11.4 cm) guns were substituted in their
place.

10) When the Ca class destroyers were modernized
in the 1950s, the mountings were upgraded with improved RPC equipment and
then designated as Mark 5* Mod 1. At the same time, these ships were
also given modern Mark 6M directors linked to Flyplane Mark 5 fire control
systems.

11) Single mountings removed from scrapped
"C" destroyers were later used on the Tribal class frigates (Type 81) built
in the 1950s. These mountings were then designated as Mark 5* Mod
2. The most noticeable difference was that the layers' and trainers'
sighting ports were plated over to improve the watertightness of the front
gun shield. This mounting was controlled by the MRS3 gunfire control
system which was a near-copy of the USN Mark 63 to which it strongly resembles.
It has been noted that the use of these open mounts on modern ASW ships
meant that RN sailors would go into battle as exposed to the elements as
were their grandfathers in World War I.

12) The gun axes in all twin mountings
were 38 in (96.5 cm) apart.

13) Renown had her six forward twin Mark
II BD mountings removed in the summer of 1945 after she had been placed
into a reduced state. These mountings were to be reused on carriers,
but this was not carried out. It is speculated that Renown was to
get new, more advanced mountings during a refit planned for late 1945,
but the end of the war halted all such work and she went to the breakers
in 1948.

14) The Mark I mount was a single UD design
not used in service. The RP10 Mark II* BD mount was intended for
the Albion class carriers but not used. Taking the previous note
into account, it is possible that these would have been the mountings removed
from Renown with some alterations.

.

Data from"Nelson to Vanguard: Warship Design
and Development 1923-1945" and "The Design and Construction of British
Warships 1939-1945: Volume I" both by D.K. Brown"Naval Weapons of World War Two" by John
Campbell"The Naval Institute Guide to World Naval
Weapon Systems 1991/92" and "British Carrier Aviation" both by Norman Friedman"Leander Class Frigate Conversions"
article by Peter Hodges in "Warship Volume III""Destroyer Weapons of World War 2" by
Peter Hodges and Norman Friedman"Warships of World War II" by H.T. Lenton
and J.J. Colledge"British Battleships of World War Two"
and "British Cruisers of World War Two" both by Alan Raven and John Roberts"The Battle-cruiser HMS Renown 1916 -
1948" by Peter C. Smith"Destroyers of World War Two" by M.J.
Whitley---H.M.S.
Solebay Association
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